1. Field of the Invention
The present invention relates to a method of controlling an electric compressor, and more specifically to a method of controlling a motor provided for an electric compressor.
2. Description of the Related Art
An electric compressor is widely used in various fields, for example, an air-conditioner, a refrigerator, etc.
An electric compressor is provided with a motor, and realizes a cooling capability by compressing a refrigerant using the rotary motion of the motor. The motor is controlled such that, for example, it can be operated at a constant speed, based on difference between a user-specified temperature and the current actual temperature, etc.
The speed of a motor (rotational speed) can be controlled basically by monitoring the position of a rotor using a position sensor such as a Hall device, etc. However, in the electric compressor, it is desired to use a system of controlling the speed of a motor by estimating the position of a rotor based on the electromotive force, current, etc. of the motor (hereinafter referred to as a sensorless system) instead of using such a position sensor. Normally, in the sensorless system, the rotational speed is given as a control instruction value, and the motor is driven such that the actual rotational speed matches the control instruction value.
However, if a compressor is left in unoperational state for a long time, then the refrigerant in gaseous form during the operation of the compressor may be liquefied and left in the compressor. When the compressor is driven in this state, the motor requires large torque. Especially when a predetermined rotational speed is given as a control instruction value in the sensorless system, and the motor is to be driven according to the control instruction value, very large torque is required and the motor is sometimes driven asynchronously. Additionally, this large torque also requires an inverter circuit with large capacity.
The method of solving the above mentioned problems with the electric compressor is described in, for example, Japanese Patent Application Laid-open No. Heisei 6-241183 (U.S. Pat. No. 5,518,373). The electric compressor described in this official gazette discharges a liquid refrigerant by operating the motor in step mode for a predetermined period at the start of driving the motor, and then enters a normal operation mode. However, this method described in the official gazette may take a long time to perform the operation of discharging the liquid refrigerant. Furthermore, although some other methods are introduced in the above mentioned official gazette, there are the problems that the compressor is large, the liquid refrigerant cannot be completely removed, and the compressor itself vibrates, etc.